1. Field of the Invention
The invention pertains to a continuous process and related apparatus for the conversion of inorganic solid starting particles which either are amorphous or possess a degree of order into solid inorganic product particles which    (a) when the starting particles are amorphous, possess a degree of order, or    (b) when the starting particles possess a degree of order, possess a different order, a different degree of order, or no order.
2. Prior art
Processes for the conversion of inorganic solid particles in the form of a suspension are known, for instance from German patent publication DE 38 23 895, which describes a process for the preparation of boehmite and alpha-aluminum oxide monohydrate compounds having variable pore radii in the range of 3 to 100 nm. In the said process suspensions containing 5 to 15 wt % Al2O3 are aged in an autoclave at a steam pressure of 1 to 30 bar, preferably for between 0.5 and 20 hours, whilst stirring at a peripheral speed of 1.0 to 6.0 m/s. The said stirring preferably takes place in a cascade reactor with 2 to 10, preferably 4 to 10 stages (as shown in FIG. 3 of DE 38 23 895).
The Solids to Liquid Ratio (SLR) in the process according to DE 38 23 895 ranges from roughly 0.05 to 0.18, which means that the suspensions used in this process are relatively large in volume and require similarly large reactors and peripheral equipment.
For many applications, e.g., catalysts, carriers, adsorbents, fillers, electronic materials and/or nano-technology applications, it is preferred to convert solid inorganic starting particles which either are amorphous or possess a degree of order into inorganic solid product particles which possess a degree of order, a different order, a different degree of order, or no order. In this specification “a degree of order” is defined as the presence of a crystalline or quasi-crystalline, i.e. non-amorphous, phase detectable by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) or extended X-ray adsorption fine structure (EXAFS). Normally, a degree of order will be X-ray detectable (either as a peak or as a lump), but in the case of very small crystallites (i.e. below the XRD detection limit) more advanced techniques are required to detect a degree of order: SEM, TEM, or EXAFS. On the other hand, amorphous is defined as not having a degree of order as defined above. The degree of order can be estimated for instance from the width of the XRD-peak (or lump) if the crystallites are X-ray detectable. The narrower this peak, the higher the degree of order. A different order will follow from the detection of different crystal structures or morphologies as detected by the techniques mentioned above. No order means amorphous.
In order to minimise the costs of operation and to maximise energy conservation, the conversion of inorganic solid starting particles is preferably carried out in a continuous mode and with the minimum of liquid required to suspend the starting particles on the one hand and ensure proper flow characteristics on the other.
Suspensions consist of a continuous phase, i.e. a liquid, and a dispersed phase, i.e. solid particles. Suspensions can be homogeneous or heterogeneous. In this specification, homogeneous suspensions are defined as suspensions having a constant volume fraction of the continuous phase throughout the whole system. Suspensions without such a constant volume fraction of the continuous phase are referred to as heterogeneous. In these heterogeneous systems there are concentration gradients of the dispersed phase.
Suspensions can separate into a fraction with a higher volume fraction of the continuous phase and a fraction with a lower volume fraction of the continuous phase. Within this specification this phenomenon is referred to as segregation. Segregation can occur by the action of various forces, for instance centrifugal forces or gravity. Sedimentation is a form of segregation where the dispersed phase settles by gravity.
When a sediment is formed, part of the flow region within a reactor is blocked by a stagnant solid, reducing the volume available for free flow. With constant mass flux, the suspension will have to move through a smaller area, resulting in higher velocities of the continuous phase. This results in even more segregation and a non-ideal residence time distribution of the dispersed phase in the reactor.
The conversion of inorganic solid starting particles in a suspension may be performed continuously in traditional pipe reactors or cascade reactors as described for instance in the aforementioned DE 38 23 895, provided that the starting particles easily form a stable homogeneous suspension, e.g., a sol or a gel, and are of a more or less uniform particle size. Even then limitations in the Solids to Liquid Ratio (SLR) may occur due to the rheological behavior of the homogeneous suspension. High energy input, e.g., high-shear mixing, may alleviate these difficulties if the suspensions exhibit shear-thinning behavior.
Unfortunately, many industrially interesting materials are not easily suspendable and/or do not form stable homogeneous suspensions at high solids to liquid ratios. This is due either to their large particle size (say >0.1 micron) or to their chemical incompatibility with the liquid, making segregation of the particles from the liquid very likely. This means that the solid particles will show a tendency to form a sediment layer, resulting in an uncontrolled and non-ideal residence time distribution in the reactor. This lack of homogeneity may hinder the conversion, especially when additional components, for instance colloidal seeds or other reactants, need to be contacted with the starting particles. This situation may be further aggravated if we are dealing with starting particles of different sizes.
Contrary to the case of the stable homogeneous suspensions described above, where high shear can assist in homogenisation and reduction of the viscosity, unstable suspensions tend to segregate even faster when a high energy input is added to the system. Therefore, good mixing throughout the whole reactor and avoiding any dead or non-mixing zones is preferred to avoid non-ideal residence time distributions and to promote efficient conversion of the starting particles.
Alternatively, expensive chemicals need to be added in order to stabilize and disperse the suspension and to prevent segregation.